Flue gases originating from oil refineries, fossil fuel based power plants, cement plants contain hazardous gaseous pollutants such as sulfur dioxide (SO2), sulfur trioxide (SO3), hydrogen sulfide (H2S), hydrogen chloride (HCl), hydrogen fluoride (HF), nitrogen oxides and carbon dioxide (CO2) that have some severe adverse short-term as well as long-term effects on human health and the environment. Different strategies have therefore been devised for reducing the amount of the afore-stated pollutants from flue gases before they are released into the atmosphere.
The amount of CO2 present in flue gas can be reduced by methods such as burning less coal, improving the efficiency of coal-fired power plants and capturing, followed by storing the captured CO2. Among the CO2 capture techniques such as pre-combustion, post-combustion and oxy-combustion etc., post-combustion is the most promising technique since it does not require any extensive rebuilding of the existing process plant.
Absorption technique such as wet scrubbing amine absorption, is another conventional technique for carbon dioxide capture. However, the absorption techniques are associated with several drawbacks such as (i) limitation in the rate of absorption of CO2 due to diffusional resistance through the liquid phase; (ii) high energy requirement for amine regeneration (2.5-4.0 GJ/ton CO2); (iii) oxidative degradation and acidification of solvent due to the presence of oxygen in the flue gas thereby making it corrosive, in addition to causing loss in the available alkalinity for carbon dioxide capture; (iv) loss of amine due to its appreciable volatility results in equilibrium losses of amine to the treated gas; and (v) thermal degradation of amine, rendering it unsuitable for continued use and hence, the requirement for substantial amounts of fresh make-up amine. Further, it is observed that the amine absorption process is restricted to carbon dioxide capture at ambient temperatures. Furthermore, capturing carbon dioxide using the absorption technique requires energy in the form of electricity or steam or both, that is supplied by process plants like power plants which reduces the overall efficiency of the power plant by up to 13%.
Therefore, there is an increased interest in developing less expensive and/or energy integrated processes for capturing carbon dioxide. The adsorption processes generally employing solid adsorptive material, that fall under the post combustion category, serve as an alternative to the absorption based process. This is because replacing water by solid support greatly reduces the energy required for carbon dioxide capture due to the lower heat capacity of solid supports as compared to water. Published literature on carbon dioxide capture by the adsorption technique shows considerable CO2 capture capacity. However, the high temperature and resource requirements for the regeneration of the capture media influence the overall cost and time efficiency of the process.
U.S. Pat. Nos. 7,731,782 and 7,947,120 disclose adsorption processes that employ zeolite for carbon dioxide capture and utilize the heat of compression of the resulting carbon dioxide rich stream in the desorption step. Here, the carbon dioxide rich stream is compressed after the adsorption-desorption step and the heat generated is used for matching the heat demand, resulting in significant requirement of energy to carry out the process.
WO 2012083108 discloses sodium carbonate enriched sorbent based adsorption process for carbon dioxide capture. The method utilizes the exothermic heat from the adsorber via heat exchange with LPG or propane as the working fluid for production of power. This requirement of additional LPG or propane adds to the costs and inventory.
U.S. Pat. No. 5,917,136 discloses a pressure swing adsorption process for carbon dioxide capture that uses modified alumina adsorbents. The modified adsorbents have carbon dioxide sorption capacities of about 0.11-0.29 mmoles/g, however, they are of expensive nature.
Most of the processes for capturing carbon dioxide from air or flue gas stream utilize heat from an external source together with the heat made available by compressing the desorbed vapor product (pure carbon dioxide). However, this type of heat utilization does not significantly increase the cost-efficiency of the process.
A need is, therefore, felt for developing a simple, energy efficient and economic process as well as a system for the removal of carbon dioxide present in flue gases that overcomes the drawbacks associated with the prior art.